Derby ended a run of three home defeats when they celebrated a second victory in five days against ten-man Watford.
The introduction of teenage winger Lee Holmes turned the match as he set up Derby's two second-half goals for Mathias Svensson and Junior.
Watford, who had Paul Devlin sent off for a second yellow card in first-half stoppage time, twice fought back to equalise but were beaten by Junior's header two minutes from time.
Derby had taken an 18th minute lead through a brilliant goal by skipper Ian Taylor but then lost key defender Michael Johnson after he collided head-first with an advertising hoarding.
The Jamaican was off the field for ten minutes and Derby paid the price for delaying the substitution when Marcus Gayle headed an equaliser.
Taylor's third goal for the club, a 22-yard right-foot volley, had given his side a deserved advantage but Watford showed their spirit by battling back and Gayle made no mistake when Lee Cook curled in a free kick from just outside the right-hand edge of the penalty area.
The dismissal of Devlin for a second foul on Luciano Zavagno gave Derby an advantage but Watford looked like frustrating them until Rams manager George Burley brought on Holmes in the 67th minute.
His pace and ability to deliver a telling cross set up Svensson in the 73rd minute for a left-foot volley after Junior had failed to connect with a header.
But Watford made a decisive substitution of their own when Scott Fitzgerald and Ashley Young came on and combined for an 81st minute equaliser.
Fitzgerald drove the ball in low from the right and Young got in between Andy Oakes and a defender to steer a right-foot shot in from six yards.
It could have been even worse for Derby when Fitzgerald powered through the middle to unleash a low shot that Oakes saved to his right but Holmes shattered Watford with two minutes remaining.
The teenager, who made history last season when he became the youngest player at 15 to appear in the FA Cup, again sent over a superb cross which Junior directed back into the roof of the net from an acute angle at the back post.